Engineering Guide
Power & Energy Conversion Guide for Engineers: kW, HP, BTU/hr, Ton, Therm & MBTU
Published June 18, 2026 · by Industrial Unit Converter Editorial Team
Why power unit fluency prevents specification errors
Power and energy units form the backbone of every equipment specification and energy analysis. A chiller rated at 500 kW in a European datasheet needs to be compared against a US specification calling for 150 tons. A boiler nameplate might read 4,000 MBH (thousands of BTU/hr) while the building automation system reports energy in kWh. The conversions are simple arithmetic — but the consequences of getting them wrong are measured in six-figure equipment purchases and years of utility bills.
This guide covers every power and energy unit an engineer encounters: the conversion formulas, the quick-reference tables, and the tools that eliminate manual calculation.
Power vs energy: the distinction in two sentences
Power is the rate of energy transfer at an instant in time. Units: watt (W), kilowatt (kW), horsepower (HP), BTU per hour (BTU/hr), ton of refrigeration.
Energy is power integrated over time. Units: kilowatt-hour (kWh), BTU, therm, joule, MBTU (million BTU).
If a 10 kW motor runs for 8 hours, it consumes 80 kWh of energy. If a 1,000 MBH boiler fires for 12 hours, it delivers 12,000 MBTU (12 million BTU).
The complete power unit conversion table
| From | To | Multiply by | Inverse |
|---|---|---|---|
| kW | HP (mechanical) | 1.341 | HP ÷ 1.341 = kW |
| HP | kW | 0.7457 | kW ÷ 0.7457 = HP |
| kW | BTU/hr | 3,412 | BTU/hr ÷ 3,412 = kW |
| BTU/hr | kW | 0.0002931 | kW × 3,412 = BTU/hr |
| HP (mech) | watts | 745.7 | W ÷ 745.7 = HP |
| kW | watts | 1,000 | Definitional |
| Ton (refrig) | kW | 3.517 | kW ÷ 3.517 = tons |
| Ton (refrig) | BTU/hr | 12,000 | Definitional |
| BTU/hr | HP (boiler) | 0.0000299 | 1 boiler HP = 33,475 BTU/hr |
| kW | MBH | 3.412 | 1 MBH = 1,000 BTU/hr |
| W | BTU/hr | 3.412 | Standard conversion |
Key conversions worth memorizing:
- 1 kW = 3,412 BTU/hr — the universal power bridge between SI and imperial
- 1 HP = 0.746 kW = 2,545 BTU/hr — motor nameplate conversions
- 1 ton of refrigeration = 12,000 BTU/hr = 3.517 kW — chiller and AC sizing
- 1 MBH = 1,000 BTU/hr — boiler ratings; 1 MMBH = 1,000,000 BTU/hr
- 1 W = 3.412 BTU/hr — for calculating heat gain from electrical equipment
Worked example: kW to HP
A 75 kW electric motor — what's the horsepower rating?
75 × 1.341 = 100.6 HP → select a 100 HP motor (standard NEMA frame size)
Worked example: chiller sizing — tons to kW
A 200-ton chiller. What's the equivalent in kW?
200 × 3.517 = 703.4 kW of cooling capacity
If the chiller has a COP of 5.0, the electrical input power is: 703.4 ÷ 5.0 = 140.7 kW
Use our BTU/hr to kW Calculator and Ton of Refrigeration to kW Converter for instant conversions.
Horsepower variants: which HP are you using?
Not all horsepower is created equal. The different HP definitions can differ by 1-5% — enough to affect motor selection and energy calculations:
| HP Type | Equivalent | Used For |
|---|---|---|
| Mechanical HP (Imperial) | 745.7 W = 550 ft·lb/s | US motors, pumps, compressors (NEMA) |
| Metric HP (PS / CV) | 735.5 W = 75 kg·m/s | European and Asian motor nameplates (DIN, JIS) |
| Boiler HP | 33,475 BTU/hr = 9.81 kW | Steam boiler ratings (1 BHP = evaporation of 34.5 lb/hr of water at 212°F) |
| Electrical HP | 746 W (exact) | Electrical power calculations |
| Hydraulic HP (WHP) | Q (GPM) × ΔP (psi) ÷ 1714 | Pump hydraulic power output |
The takeaway: A "10 HP" European pump with a metric motor actually delivers 7.35 kW shaft power, while a "10 HP" US pump delivers 7.46 kW — a 1.5% difference. For specification comparisons across international boundaries, always convert to kW.
Use our Water Horsepower Calculator for pump hydraulic power calculations and our Compressed Air Flow to HP tool for compressor sizing.
Energy units: kWh, BTU, therm, MBTU
While power conversions dominate equipment specifications, energy conversions dominate utility bills and energy audits:
| Unit | Equivalent in kWh | Equivalent in BTU | Typical Use |
|---|---|---|---|
| 1 kWh | 1.0 | 3,412 | Electricity billing (universal) |
| 1 therm | 29.3 | 100,000 | US natural gas billing |
| 1 MBTU (thousand BTU) | 0.2931 | 1,000 | Gas equipment nameplates (MBH input) |
| 1 MMBTU (million BTU) | 293.1 | 1,000,000 | Large gas billing, boiler plants ("MM" = Roman M × M = thousand × thousand) |
| 1 GJ (gigajoule) | 277.8 | 947,817 | Canadian and European natural gas billing |
| 1 gallon #2 fuel oil | 40.6 | 138,500 | Fuel oil energy content |
| 1 CCF (hundred cubic feet) of natural gas | 29.3 | 100,000 | 1 CCF ≈ 1 therm at standard heat content |
Converting between energy units
kWh to BTU: kWh × 3,412 Therms to kWh: therms × 29.3 MMBTU to kWh: MMBTU × 293.1 kWh to therms: kWh ÷ 29.3
Worked example: natural gas billing
A facility burns 8,500 therms of natural gas per month. What's this in kWh?
8,500 × 29.3 = 249,050 kWh per month
At $0.10/kWh electricity and $1.00/therm natural gas:
- Gas cost: 8,500 × $1.00 = $8,500/month
- Equivalent electricity cost: 249,050 × $0.10 = $24,905/month
The fuel cost ratio of 2.9:1 is typical for gas vs electric heating and drives most decisions about dual-fuel and heat pump economics.
Common conversion workflows in engineering practice
1. Chiller plant design (tons ↔ kW)
A cooling plant with three 400-ton chillers: total capacity = 1,200 tons.
- In kW: 1,200 × 3.517 = 4,220 kW
- Annual energy at 0.6 kW/ton average efficiency, 4,000 equivalent full-load hours: 4,220 × 0.6 × 4,000 = 10,128,000 kWh/year
- At $0.10/kWh: $1,012,800/year in chiller plant electricity
2. Boiler replacement (MBH ↔ kW)
Replace a 4,000 MBH atmospheric boiler (80% efficient) with condensing boilers (95% efficient):
- Input rating: 4,000 MBH = 4,000 × 1,000 = 4,000,000 BTU/hr
- Output (old): 4,000,000 × 0.80 = 3,200,000 BTU/hr
- Output (new): 4,000,000 × 0.95 = 3,800,000 BTU/hr
- Fuel saved: (3,800,000 − 3,200,000) / 0.80 efficiency = 750,000 BTU/hr input reduction = 750 MBH fuel savings
3. Data center PUE calculation (kW → BTU/hr → tons)
A data center has 500 kW of IT load and a PUE of 1.4:
- Total facility power: 500 × 1.4 = 700 kW
- IT heat = 500 × 3,412 = 1,706,000 BTU/hr
- Total heat to reject: 700 × 3,412 = 2,388,400 BTU/hr
- Cooling required: 2,388,400 ÷ 12,000 = 199 tons
Use our kW to BTU/hr and BTU/hr to Watts tools for daily conversions.
Quick conversion formulas for field use
When a calculator is not handy, these approximations are accurate to within 2%:
| Conversion | Exact | Approximate (Mental Math) |
|---|---|---|
| kW → HP | × 1.341 | × 4/3 (−0.3%) |
| HP → kW | × 0.7457 | × 3/4 (+0.6%) |
| kW → BTU/hr | × 3,412 | × 3,400 (−0.4%) |
| BTU/hr → kW | ÷ 3,412 | ÷ 3,400 (+0.4%) |
| Tons → kW | × 3.517 | × 3.5 (−0.5%) |
| kW → tons | ÷ 3.517 | ÷ 3.5 (+0.5%) |
| HP → BTU/hr | × 2,545 | × 2,500 (−1.8%) |
| kWh → BTU | × 3,412 | × 3,400 (−0.4%) |
| Therms → kWh | × 29.3 | × 30 (+2.4%) |
Common power conversion mistakes
1. Mixing boiler HP and mechanical HP
1 boiler HP = 33,475 BTU/hr = 9.81 kW = 13.15 mechanical HP. A specification calling for a "10 HP boiler" is almost certainly referring to boiler HP (334,750 BTU/hr), not mechanical HP (25,450 BTU/hr). The difference is a factor of 13. Always verify which HP is intended.
2. Applying the COP only to input power, not output
A 100-ton chiller with a COP of 5.0 requires 100 × 12,000 / 5.0 = 240,000 BTU/hr electrical input = 70.3 kW electrical, not 100 × 3.517 / 5.0 = 70.3 kW (same answer either way — but the formula must use the correct numbers: tons × 12,000 / COP = BTU/hr input).
3. Confusing MBH and MMBH
MBH = thousands of BTU/hr (kBtu/hr). MMBH = millions of BTU/hr. The "M" in MBH comes from the Roman numeral M = 1,000. The "MM" in MMBH is M × M = 1,000 × 1,000 = 1,000,000. A 4,000 MBH boiler = 4 MMBH boiler. Many spec errors arise from this confusing notation.
4. Neglecting part-load efficiency in annual energy estimates
Equipment efficiency at full load (rated) differs from efficiency at part load (typical). A chiller rated at 0.55 kW/ton at full load might achieve 0.45 kW/ton at 60% load. Using the full-load efficiency for annual energy estimates over-predicts consumption by 10-20%.
Engineering standards and references
- ANSI/ASHRAE Standard 90.1 — Energy Standard for Buildings (defines equipment efficiency metrics: EER, SEER, COP, IPLV)
- AHRI Standard 550/590 — Performance rating of water-chilling and heat pump water-heating packages
- NEMA MG 1 — Motors and generators (defines mechanical HP rating, efficiency classes)
- ISO 80000-4 — Quantities and units — Part 4: Mechanics (defines SI power and energy units)
For all industrial power and energy conversions, explore our complete set of calculators: BTU/hr to kW, kW to BTU/hr, BTU/hr to Watts, Ton of Refrigeration to kW, and the HVAC Conversions hub.
Summary
| Concept | Rule |
|---|---|
| Power vs energy | Power (rate): kW, HP, BTU/hr. Energy (total): kWh, BTU, therm |
| kW ↔ HP | 1 kW = 1.341 HP mech; 1 HP mech = 0.7457 kW |
| kW ↔ BTU/hr | 1 kW = 3,412 BTU/hr; 1 BTU/hr = 0.0002931 kW |
| Tons ↔ kW | 1 ton refrig = 12,000 BTU/hr = 3.517 kW |
| Energy conversions | 1 kWh = 3,412 BTU; 1 therm = 100,000 BTU = 29.3 kWh |
| Boiler HP | 1 BHP = 33,475 BTU/hr — not the same as mechanical HP |
| Motor efficiency | NEC Table 430.250 provides FLC; actual power = FLC × V × √3 × PF × Eff |
| MBH vs MMBH | MBH = 1,000 BTU/hr; MMBH = 1,000,000 BTU/hr. 1 MMBH = 1,000 MBH |
Last reviewed: June 2026. Conversion factors per NIST Special Publication 811. Chiller ratings per AHRI 550/590. Motor ratings per NEMA MG 1.